I would be suprised that a leaking port could leak enough to fill the hull in 30 mins. Could it be that there was already a lot of water in the hull before you went sailing?
Most of those ports - called inspection ports have an o-ring seal. Problem is I have never seen only the o-rings (or enven just the covers for sale separately so you may have to replace the whole port. The good news is that you can get the whole thing for under $20.